Pneumatic actuating drives are known to include an electronic position regulator to regulate a desired opening level of a fitting, which is connected to the pneumatic actuating drive, on the basis of a predetermined nominal value. By way of example, the fitting can be a process valve within a pipeline system of a process installation. In addition, pneumatic actuating drives can also be used for operation of other industrial fittings and the like.
The product prospectus “Der kompakte, intelligente Stellungsregler” [The compact, intelligent position regulator] (ABB Automation Products GmbH, document number: 50/18-19 DE RevA, June 2005 edition) discloses an electronic position regulator for a pneumatic actuating drive. The position regulator is in the form of a type of electronics box which can be fitted to the outside of the pneumatic actuating drive. The position regulator is a configurable appliance which can communicate, such as by means of a field bus. A functional aspect is the microprocessor-controlled procedure for a regulation program. The nominal value can be preset via a field bus connection which is designed using two-conductor technology. Furthermore, the position regulator has a supply air connection for an air pressure up to a maximum of 6 bar, as well as a working connection for passing on the control pressure generated by the position regulator to the control chamber of the pneumatic position regulator.
In addition, the position regulator has a sensor input for supplying the actual value of the present position of the switching element, which is operated by the pneumatic actuating drive. The actual value of the present position is obtained by means of a position sensor, which is arranged on the switching element. The pneumatic drive for the actuating drive is provided continuously by an I/P module with a downstream 3/3-way valve. The 3/3-way valve controls the passage for ventilation or venting of the actuating drive as proportionally as possible. A closed position, in which all the external connections are blocked, is assumed in a mid-position. The configuration and observation of the operating state of the position regulator can be carried out either by a built-in control panel directly in situ, or centrally via a communication connection, on the basis of the bus protocol by means of a superordinate control unit.
US 2007/0045579 A1 discloses a pneumatic position regulator which is in the form of an I/P module with a 3/3-way valve. The 3/3 switching function allows the switch positions of ventilation, closed position and venting of a working connection, which supplies the control pressure for the connected pneumatic actuating drive. The electropneumatic valve with a 3/3 switching function has two closure elements, which point in mutually opposite directions of the actuating movement and act with the same magnitude with respect to one another. Each of the closure elements bounds an internal control chamber, with a common control pressure connection being associated with the two control chambers. While one closure element is used for ventilation of the working connection, the other closure element is used for venting of the working connection. When neither of the two closure elements is operated, then the valve is in the closed position.
For position regulation, an electropneumatic valve is intended to provide as proportional a response as possible for the electrical drive signal with respect to the pneumatic manipulated variable of the switching element that is supplied, with disturbance variables, which are caused by the forces on the switching element of the fitting and the hysteresis, reacting on the pneumatic side. Furthermore, influencing variables such as temperature fluctuations, pressure fluctuations and the like, can disturb the ideal proportionality ratio. In order to achieve a response which is as linear as possible, the position regulator in some cases takes into account correction values that are determined using sensors. The results which can be achieved in this way are, however, generally not satisfactory. A hysteresis of the valve mechanism forms a significant influencing factor in this case.
Until now, according to known techniques, this problem has been solved by signal processing involving carrying out friction suppression of the sliding friction by means of a fixed correction factor. However, in the starting phase of the valve mechanism from a rest position, the static friction is dominant, and this has not been adequately compensated for.